1. Field of the Invention
This invention relates generally to image display systems and more particularly to a method and apparatus for flicker filtering image data for presentation on a display screen through interlaced scanning.
2. Description of the Related Art
Images presented on the display screen of a television through interlaced scanning are presented in a manner that ensures that the image has an even brightness throughout instead of having separate bright and dark bands. With interlaced scanning, each still picture is made up of two scans consisting of alternate lines. For example, the even numbered lines of one field are scanned and then the odd numbered lines of a second field are scanned. A complete pass through all the even and odd numbered lines make-up one frame of data.
Sharp transitions in color between adjacent scan lines result in flicker when presented on a television screen. That is, flicker is caused on a National Television System Committee (NTSC) monitor screen when a pixel or horizontal group of pixels of a higher intensity are bordered on the top or bottom by lower intensity pixels. The flicker causes a strain on human eyes as well as deterring from the quality of the display. As bandwidth is a concern with television, interlaced scanning has been adopted to allow the frame repetition rate to be reduced by one half. As mentioned above, an interlaced scan is made up of a frame of data that is divided into an even field and an odd field, wherein a single vertical scan covers half the lines, i.e., the even field or the odd field. Thus, each frame takes two vertical scans with even and odd lines scanned on alternate fields.
FIG. 1 is a schematic diagram of a process for filtering interlaced image data to reduce flicker. Memory 100 contains image data stored as even and odd lines. Even data is placed into even pipe 102 through one memory access, while odd data is placed into odd pipe 104 through a second memory access. Corresponding even and odd data is then filtered through flicker filter 106. Flicker filter 106 averages the corresponding even and odd data to reduce flickering between adjacent lines presented on display screen 110. Where display screen 110 is a television screen, the display screen will typically have 525 lines with half of the lines in an even field and the other half in an odd field. Depending on the flicker filter algorithm employed, incoming data can be written to memory 100 twice.
Furthermore, the lines of distinction between computers and televisions are blurring, since computers are taking on characteristics of televisions and televisions are taking on characteristics of computers. However, the displays for a television are presented through interlaced scanning while computers use progressive scanning techniques. Interlaced scanning techniques are well suited for television displays because they are less demanding on the memory of a television, which is limited relative to a computer's memory resources. Moreover, as television systems further enhance the functionality available to the viewer and as television manufacturers simultaneously embrace high definition television standards, the demands on memory will further rise. In turn, costs are incurred in supporting these demands, such as increased memory capacity to avoid image corruption. Additionally, the increased memory capacity corresponds to greater power consumption.
As a result, there is a need to solve the problems of the prior art to provide a method for reducing the demands on memory and corresponding power required by the memory while maintaining the quality of a display of image data generated through interlaced scanning.